1. Field of the Invention
The present invention relates to an image forming apparatus using an electrophotographic system or an electrostatic recording system and, more particularly, to an image forming apparatus such as a copying machine, printer, or a facsimile machine.
2. Related Background Art
Conventionally, a two-component developer having toner and carrier particles as main components are used in developing devices provided in image forming apparatuses using an electrophotographic system or an electrostatic recording system. It is preferred from the viewpoint of image hues or tones that a two-component developer should be used, in particular, in a color image forming apparatus for forming a full-color or multicolor image by an electrophotographic system. As is well known, the density of toner (i.e., the proportion of the weight of toner particles to the total weight of carrier and toner particles) of a two-component developer is a particularly important factor in stabilizing image quality. Toner particles in a developer are consumed during development to change the toner density in a developing device (developing container) and there is, therefore, a need to perform toner density control for maintaining the toner density in a suitable range to ensure the desired image quality. For this control, a toner density controller (automatic toner replenisher (ATR)) is used which accurately detects at suitable intervals the density of the developer (toner) provided in the developing device and supplies toner according to a change in the toner density.
Various methods have been proposed and put to practical use as a method for detecting the toner density in a developing device and a method for controlling the developer density for the purpose of correcting a change in toner density due to consumption during development, i.e., for controlling the amount of toner supplied to a developing device.
For example, a method (1) is known which uses a toner density controller having a detection means arranged close to a development sleeve ordinarily used as a developer bearing member or a developer transport path in a developing container to detect an amount of a developer on the development sleeve or light reflected by a developer in the developing container when the developer is irradiated with light. The toner density controller detects and controls the toner density by utilizing a phenomenon in which the intensity of reflected light changes depending on the toner density.
A method (2) is also known which uses an inductance detection type of toner density controller having an inductance head provided on, for example, a side wall portion of a developing container as an inductance sensor for detecting the apparent permeability according to the mixture ratio of a magnetic carrier and a nonmagnetic toner and for converting the apparent permeability into an electrical signal. The actual toner density in the developer in the developing container is detected through the detection signal from the inductance head and is compared with a reference value, and toner is supplied according to the result of this comparison.
Further, a method (3) is known in which the density of a patch image formed on a cylindrical electrophotographic photosensitive member, i.e., a photosensitive drum, ordinarily used as an image bearing member is detected by using a light source provided at such a position opposite to the surface of the photosensitive drum and a sensor for receiving light reflected from the surface. A signal representing the patch image density is converted into a digital signal. by an analog-to-digital converter, and the digital signal is supplied to a central processing unit (CPU) and compared with an initial set value by the CPU. If the density is higher than the initial set value, supply of toner is stopped until the density becomes equal to the initial set value. If the density is lower than the initial set value, forced toner supply is continued until the density becomes equal to the initial set value. The toner density is thus controlled indirectly to be maintained at the desired value.
Methods in which the density of toner is detected from the reflectivity of a developer transported onto a development sleeve or contained in a developing container while the developer is being irradiated with light, as in the above-described method (1), entail a problem that accumulation of a contaminant, e.g., scattered toner on the detection means reduces the accuracy with which the toner density is detected. Methods in which the toner density is indirectly controlled through the density of a patch image, as in the above-described method (3), entails a problem that the difficulty in securing the space for formation of the patch image and the space for placement of the detection means is increased with the reduction in size of image forming apparatuses designed as copying machines or the like.
In contrast, inductance detection methods, typified by the above-described method (2), are considered to be most effective in realizing an image forming apparatus designed as a space-saving type manufactured at a low cost, because the sensor unit is low priced and because they are free from the problem of contamination of a detection means due to scattering of toner and the space problem described above with respect to the methods (1) and (3).
The inductance detection type of toner density controller (inductance detection type of ATR) controls the toner density on the basis of a control method such that, when it is determined as a detection result that the apparent permeability of the developer is large, supply of toner is started because the detection result signifies that the toner density has become lower while the proportion of the volume occupied by carrier particles in a certain volume of the developer has been increased, and that, when the apparent permeability is reduced, supply of toner is stopped because the detection result signifies the toner density has become higher while the proportion of the volume occupied by carrier particles in the certain volume of the developer has been reduced.
On the other hand, corona chargers have generally been used as charging means (charging device) for charging an image bearing member such as an electrophotographic photosensitive member or an electrostatic recording dielectric member in electrophotographic type or electrostatic recording type of image forming apparatuses. In recent years, however, contact-type charging devices, i.e., charging devices based on a method of charging a member to be charged with electricity (hereinafter referred to as a xe2x80x9ccharged memberxe2x80x9d) by bringing a charging member, to which a voltage is applied, into contact with the charged member, have been put to practical use because of their advantage of limiting generation of ozone and power consumption. In particular, charging devices of a roller charging type having a charging roller as a charging member are preferred by considering charging stability and are being advantageously used.
In the charging system using the roller charging type of charging device, however, the electrical resistances of the charging roller and the image bearing member change under the influence of environmental changes since charging is performed by discharge from the charging member to the charged member, resulting in variation in the surface potential of the image bearing member.
A charging method which reduces the influence of environmental changes, e.g., the one described in Japanese Patent Application Laid-Open No. 7-5748, which is a counterpart of U.S. Pat. No. 5,606,401, has recently been proposed in which a photosensitive member having a charge injection, layer formed in its surface and a conductive powder (SnO2 or the like) for a trap level dispersed in the charge injection layer is charged in a contact charging manner through an electroconductive contact-type charging member (a charging fur brush, a charging electromagnetic brush, a charging roller, or the like) to which a voltage is applied to inject charge into the: photosensitive member with the same polarity as the potential of the same.
This injection charging method does not use discharge and therefore has the advantage of limiting the applied voltage to a level not significantly different from the potential of the photosensitive member and avoiding generation of ozone which reduces the life of the photosensitive member, as well as the advantage of reducing the environment dependence.
In the case of contact charging using discharge, in order to enable the charged member to have the desired charging potential Vs, it is necessary to apply to the charging member a voltage determined by adding a discharge initiation voltage Vth (a voltage applied to the contact-type charging member when a dc voltage is applied to the contact-type charging member to start charging the charged member) to the desired charging potential Vs, i.e., a dc bias Vs+Vth. In contrast, in charge-injection charging, the charging potential Vs can be obtained as substantially the same potential as the dc bias applied to the charging member, so that the charging power source can be provided at a reduced cost.
As a contact-type charging member in such a charge injection system, a magnetic brush type of charging member or a fur brush type of charging member is preferably used by considering charging stability, contact stability, etc.
The magnetic brush type of charging member has a magnetic brush (charging magnetic brush) formed by magnetically binding conductive magnetic particles (hereinafter referred to as xe2x80x9ccharging carrierxe2x80x9d) on a supporting portion, which is also used as a feeder electrode. The charging magnetic brush is brought into contact with the charged member and a voltage is applied to the supporting portion. More specifically, the magnetic-brush-type charging member is formed in such a manner that charging carrier is magnetically bound directly on a magnet or on a sleeve containing a magnet, provided as a supporting portion, so as to form a magnetic brush in which the charging carrier stands like the ears of rice. The magnetic brush portion of this magnetic-brush-type charging member is brought into contact with the charged member while the charging member is stopped or being rotated, and a voltage is simultaneously applied to the charging member, thereby charging the charged member.
The fur-brush-type charging member has a brush portion (fur brush portion) formed by conductive fibers supported on a supporting portion, which is also used as a feeder electrode. The conductive fiber brush portion is brought into contact with the charged member and a voltage is applied to the supporting portion.
The charging performance of the fur-brush-type charging member becomes poor if its fibers are flattened during long-time use or long-term shelving. Also, the charging uniformity is liable to lower due to a restriction on the brush fiber diameter, etc. In contrast, the magnetic-brush-type charging member is free from the occurrence of the above-described phenomena relating to the fur-brush-type charging member and is capable of uniform and stable charging.
However, image forming apparatuses having a magnetic-brush-type charging device using the above-described magnetic-brush-type charging member have problems described below.
That is, during use of the magnetic-brush-type charging device, there is a possibility of the charging carrier in the magnetic brush portion of the magnetic-brush-type charging member flowing out by being attached to the image bearing member, e.g., a photosensitive drum. In the case of a developing device which controls the toner density with an inductance detection type of toner density controller, if the charging carrier attached to the image bearing member to flow out is collected in the developing device, it is gradually accumulated in the developing device as the image forming operation is repeated, although the amount of the charging carrier attached to the photosensitive drum to flow out may initially be not so large as: to immediately influence the quality of images formed by the image forming apparatus. If magnetic particles (hereinafter referred to as xe2x80x9cdeveloping carrierxe2x80x9d) used in the developing device and the charging carrier differ in permeability, the apparent permeability of the developer as a whole may change to cause an error in toner density control performed by the inductance detection type of toner density controller.
That is, if the permeability of the charging carrier is larger than that of the developing carrier, mixing of the charging carrier into the two-component developer in the developing device causes the inductance sensor to output a detection result indicating an increase in the average permeability of the developer even when the toner density in the developing container is constant. This detection result corresponds to the case where the proportion of the carrier particles in a certain volume of the developer is increased and the toner density is reduced, and supply of toner is then started. In this case, the result of density control is that the toner density is higher than the correct toner density value.
Conversely, if the permeability of the charging carrier is smaller than that of the developing carrier, mixing of the charging carrier into the two-component developer in the developing device causes the inductance sensor to output a detection result indicating a reduction in the average permeability of the developer even when the toner density in the developing container is constant. This detection result corresponds to the case where the proportion of the carrier particles in a certain volume of the developer is reduced and the toner density is increased, and the inductance detection type of ATR stops supply of toner. In this case, the result of density control is that the toner density is lower than the correct toner density value.
In the former case, an excessive supply of toner results in an excessively high image density, and there are other problems stemming therefrom: a problem of the developer increased in total amount with the increase in the amount of toner flowing out of the developing container, and a problem of the possibility of scattering of toner being increased by a reduction in the amount of charge on toner with the increase in the proportion of toner in the developer. In the latter case, a reduction in the amount of toner in the developer results in a deterioration in image quality, a considerable reduction in image density, etc. These problems may become more serious as the image forming process is repeated over many times.
Studies made by the inventor of the present invention have revealed that in the case of an image forming apparatus arranged to form an image by using a plurality of developing devices, the amount of charging carriers attached to photosensitive drums varies depending upon a characteristic of developers respectively used in the developing devices (e.g., the amount of charge per unit toner weight) and upon environmental conditions (temperature, humidity) under which the developing devices are used.
For example, in the case of an image forming apparatus which forms a toner image by using developing devices for processing on a plurality of photosensitive drums, toner remaining on the photosensitive drum without being transferred to a recording member in each transfer section, i.e., transfer residue toner, can mix in a component of the magnetic-brush-type charging member of the charging device. The resistance of the magnetic-brush-type charging member is thereby increased to reduce the efficiency of charge injection from the magnetic-brush-type charging member into the photosensitive drum surface layer. In such a situation, the charging carrier can be easily attached to the photosensitive drum to flow out.
The efficiency of transfer of toner images formed on the photosensitive drums to the recording member varies slightly depending upon the developing devices (the characteristics of the developers (the above-mentioned toner charge amount), etc.) and the transfer device. Consequently, the amount of toners remaining on each photosensitive drum varies, so that the amounts of the charging carriers attached to the photosensitive drums and flowing out is different in each of the image forming sections (developing devices).
Further, the resistances of the charging carrier and the photosensitive drum are also increased under a low-humidity condition to cause a reduction in the efficiency of charge injection from the charging carrier to the photosensitive drum surface layer (the surface layer for injection charging), such that the charging potential of the photosensitive drum surface layer is lower than the bias applied to the charging device. A potential difference is thereby caused between the charging carrier tips and the photosensitive drum surface layer to render the charging carrier liable to be attached to the photosensitive drum. Thus, the amount of the charging carrier attached to the photosensitive drum and flowing out varies depending upon environmental conditions.
In view of the above-described circumstances, an object of the present invention is to provide an image forming apparatus capable of optimizing the amount of a developer supplied to each of a plurality of development means even if magnetic toner particles mixes in a component of the development means.
Other objects of the present invention will become apparent upon reading the following detailed description of embodiments of the invention.